This invention relates to altimeters, FM-CW (frequency modulated continuous wave) and more specifically to single antenna FM altimeters.
As is well known, there are generally two types of altimeter systems, one known as a pulse radar altimeter and the other as a FM-CW radio altimeter. In the pulse radar altimeter, a series of RF pulses are transmitted towards the ground and a receiver is operated to detect the return of the transmitted pulses that are reflected from the ground. The time delay between the transmitted and received pulses is proportional to the distance to the ground. By accurately controlling the transmission and detection of such pulses, highly accurate information can be obtained with respect to distance to the ground.
In a FM-CW radio altimeter system, a continuous RF signal is repetitively swept by a frequency-modulating signal in a transmitter and the resulting modulated carrier is transmitted towards the ground. The FM signal is reflected by the ground and returned toward a receiver for reception. A portion of the transmitted signal is mixed with the reflected signal to produce a beat frequency indicative of the distance from the transmitter to the ground. By controlling the modulation of the transmitted signal and by detecting the beat frequency, accurate readings of the distance to the ground can be determined.
Aircraft FM-CW radio altimeters typically operate at altitudes from 0 to 8000 feet utilizing separate antennas for transmit and receive. Two-antenna FM-CW radio altimeters have several disadvantages. One disadvantage is the high cost and weight of cables and antennas especially in double or triple redundant radio altimeter systems on such aircraft as commercial airliners. With two antennas, multipath or double bounce is an additional disadvantage. With double bounce strong signals from the transmit antenna reflect from the ground to the fuselage of the aircraft, back to the ground and back again to the receiver antenna giving an altitude indication about twice the actual altitude. Another disadvantage with a two antenna FM-CW radio altimeter system is the requirement for high isolation between transmitter and receiver antennas resulting in rigid requirements for antenna side lobe suppression and for the antenna locations on the aircraft fuselage. The spacing between the two antennas must be large to obtain high isolation. However, this results in poor accuracy at low altitudes of the order of meters because the distance between the transmitter and receiver antenna is greater than the aircraft altitude. The spatial diversity of two separate transmitter and receiver antennas results in their isolation of no better than 75-90 dB, which restricts the maximum working altitude of an altimeter due to leakage between the antennas. Amplitude discrimination is typically used in a FM-CW altimeter to keep the system from. locking on to the leakage signal instead of the real signal. Many problems have occurred in the past with antennas failing or cable connectors loosening or degrading to cause the leakage signal to rise above the desired altitude return signal. These problems may result in the altimeter system ramping down to zero or some low altitude when an aircraft is actually at cruise altitudes ( greater than 30,000 feet).
Some of the disadvantages of a two-antenna FM-CW system may be eliminated with a single antenna system. A single-antenna aircraft FM-CW altimeter system is disclosed in U.S. Pat. No. 4,739,330. This disclosed single-antenna FM-CW altimeter operates in a pulse mode where a transmitter transmits for a short period and then a transmit-receive switch switches the antenna to a receiver for another short period. This approach has several disadvantages that limit its utilization as an aircraft altimeter. Typical aircraft FM-CW altimeter requirements are to measure altitudes from 0 to 8000 feet with an accuracy of xc2x11.5 ft (46 cm) for low altitudes and xc2x12% for high altitudes. At the low altitudes, these requirements are difficult to meet with the pulse mode of the disclosed system. In the pulse mode altitude measurements near 1.5 feet require a pulse width of about 3 nanoseconds with a repetition rate of near 300 MHz resulting in a receiver bandwidth of 1.5 GHz. These parameters cannot be easily realized in an aircraft radio altimeter with low cost, weight, and size.
Also in the aforementioned patent in the pulse mode during receive the transmitter is disconnected from the antenna with the transmit receive switch but the transmitter continues to operate at maximum power consuming twice the necessary power. With the transmitter still operating at full power in the receive mode, leakage of the transmitter signal will interfere with the receiver especially at high altitudes, where the return signal is the lowest level.
With a single antenna radio altimeter in a non-pulse or continuous wave (CW) mode, the transmitter and receiver are both operating simultaneously. The single antenna can never be perfectly matched to free space and there will always be some transmitter signal reflected back toward the receiver. At the low altitudes, leakage of a CW transmitter signal reflected from the mismatched antenna or from other elements between the transmitter and the single antenna into receiver channel is a problem that limits the performance of the altimeter.
What is needed is a single antenna FM radio altimeter system that provides improved performance over two antenna radio altimeter systems while solving the attendant problems of a single antenna altimeter.
A FM radio altimeter with a single antenna capable of operating in a continuous wave mode and an interrupted continuous wave mode to provide an altitude indication is disclosed. The single antenna FM radio altimeter includes a transmitter for generating a continuous wave altimeter signal with a constant FM modulation period below a critical altitude and an interrupted continuous wave altimeter signal with a variable FM modulation period above a critical altitude. A circulator is connected to the transmitter for providing isolation and for coupling the continuous wave altimeter signal and the interrupted continuous wave signal to the single antenna. The single antenna is connected to the circulator for radiating the continuous wave altimeter signal and the interrupted continuous wave altimeter signal and receiving a reflected continuous wave altimeter signal and a reflected interrupted continuous wave altimeter signal. A receiver is connected to the circulator for receiving the reflected continuous wave altimeter signal and the reflected interrupted continuous wave altimeter signal and for providing an altimetric beat frequency signal. A processing function compares the altimetric beat frequency signal from the receiver to a critical altitude reference beat frequency and switches the altimeter from the continuous wave mode when the altimetric beat frequency signal is below the critical altitude reference beat frequency to the interrupted continuous wave mode when the altimetric beat frequency signal is above the critical altitude reference beat frequency. The processing function provides the transmitter a constant period modulation signal below the critical altitude and a variable FM modulation period signal above the critical altitude. The processing function provides the altitude indication from the altimetric beat frequency signal below the critical altitude and from the variable FM modulation period signal above the critical altitude.
The single antenna FM radio altimeter transmitter further comprises a voltage controlled oscillator for providing a continuous wave altimeter signal with the constant FM modulation period below the critical altitude and a with the variable FM modulation period signal above the critical altitude. A coupling element is connected to the voltage-controlled oscillator for providing a local oscillator signal coupled from the continuous wave altimeter signal. A power amplifier is connected to the coupling element for amplifying the continuous wave altimeter signal. A pulse modulator is connected to the power amplifier for modulating the power amplifier to provide the interrupted continuous wave altimeter signal above the critical altitude.
The single antenna FM radio altimeter receiver further comprises a preselector connected to the circulator for amplifying and filtering the reflected continuous wave altimeter signal and the reflected interrupted continuous wave altimeter signal. A mixer is connected to the preselector for mixing the reflected continuous wave altimeter signal and the reflected interrupted continuous wave altimeter signal with the local oscillator signal to provide the altimetric beat frequency signal. A fourth switch is connected to the mixer and the coupling element for switching the local oscillator signal provided by coupling element. A beat frequency amplifier is connected to the mixer for amplifying the altimetric beat frequency signal. A filter is connected to the beat frequency amplifier for filtering parasitic signals. A dynamic selector is connected to the filter for selecting a true altitude indication.
The single antenna FM radio altimeter processing function further comprises a comparator connected to the dynamic selector for comparing the altimetric beat frequency signal to a critical altitude reference beat frequency. The comparator switches the altimeter from a continuous wave altimeter signal when the altimetric beat frequency signal is below the critical altitude reference beat frequency to an interrupted continuous wave altimeter signal when the altimetric beat frequency signal is above the critical altitude reference beat frequency. The comparator also compares the variable FM modulation period signal to a critical altitude reference modulation period and switches the altimeter from the interrupted continuous wave altimeter signal when the variable FM modulation period is above the critical altitude reference modulation period to the continuous wave altimeter signal when the variable FM modulation period is below the critical altitude reference modulation period. A searching and tracking network is connected to the dynamic selector for generating a correction signal in the interrupted continuous wave mode. A FM period regulator is connected to the searching and tracking network for providing a FM period control signal in accordance with the correction signal in the interrupted continuous wave mode. A first switch is connected to the FM period regulator and the modulator for providing the FM period control signal in the interrupted continuous wave mode and a constant modulation period control signal in the continuous wave mode. A third switch is connected to the dynamic selector and the comparator for selecting the altitude indication from the dynamic selector in the continuous wave mode and from the FM period regulator in the interrupted continuous wave mode.
It is an object of the present invention to provide a FM radio altimeter that will operate with one antenna.
It is an object of the present invention to improve isolation between the transmitter and receiver in a FM radio altimeter.
It is a feature of the present invention to provide improved accuracy at low altitude by eliminating one antenna in a FM radio altimeter system.
It is an advantage of the present invention to lower cost and weight of a FM radio altimeter system by eliminating an antenna and its associated cables.